Method of making die castings having multi-layer coated surfaces

ABSTRACT

This invention relates to a method for producing coated die castings, for example, die cast cylinders or housings for rotary or reciprocating internal combustion engines. The method is particularly directed to the production of a casting having a multiple layer coating wherein the layer exposed to the working surface of the cylinder or housing is of a material which will provide a very hard, wear resistant layer, and the underlayer, between the die casting and the wear resistant layer is of a softer, less expensive, highly adherent and physically strong material.

United States Patent [191 Bauer 3 [4 1 Mar. 19, 1974 METHOD OF'MAKINGDIE CASTINGS HAVING MULTI-LAYER COATED SURFACES [75] Inventor: Alfred F.Bauer, Toledo, Ohio [73] Assignee: NL Industries, Inc., New York, NY.

[22] Filed: Nov. 27, 1972 [21] Appl. N0.: 309,753

[52] US. Cl 29/527.3, 29/5276, 164/9,

164/33, 164/132, 164/46 [51] Int. Cl B22d 19/08 [58] Field of Search164/9, 14, 33, 46, 58,

[56] References Cited UNITED STATES PATENTS 2,074,007 3/1937 Wissler29/527;6 x 3,083,424 4/1963 Bauer 164/112 8/1963 Piper 164/69 3,401,736lmagawa 164/95 3,433,284 3/l969 Webbere et al. 164/33 X 3,689,986 9/1972Takahashi et al 29/5276 Primary Examiner-R. Spencer Annear Attorney,Agent, or FirmAllen Owen; John C. Purdue [57] ABSTRACT This inventionrelates to a method for producing coated die castings, for example, diecast cylinders or housings for rotary or reciprocating internalcombustion engines. The method is particularly directed to theproduction of a casting having a multiple layer coating wherein thelayer exposed to the working surface of the cylinder or housing is of amaterial which will provide a very hard, wear resistant layer, and theunderlayer, between the die casting and the wear resistant layer is of asofter, less expensive, highly adherent and physically strong material.

7 Claims, 5 Drawing Figures 1 METHOD OF MAKING DIE CASTINGS HAVINGMULTI-LAYER COATED SURFACES The method of the present invention is animprovement of the method disclosed in my US. Pat. No. 3,083,424, issuedApr. 2, 1963.

BACKGROUND OF THE INVENTION In my prior patent, there is disclosed amethod of making a coated cylinder or housing for an internal combustionengine in which the die casting process is used and which produces, forexample, an aluminum cylinder or housing having a wear resistant layerof a metal of much higher melting point than the aluminum and exhibitingimproved wearresistant properties. The technique has become known in theart as the transplant coat process. In the prior patent the suggestedwear resistant layer comprised stainless steel or other predominantlyiron alloys. The coating thickness suggested and disclosed in the priorpatent was from 0.015 inch to about 0.025 inch, although thickercoatings could readilybe used. Briefly stated, the former processconsists of spraying, with a metalizing gun, or otherwise depositing ona cylindrical core portion of a die, a body of metal having a highermelting point than the body of the cylinder or housing of which thecasting was to be made. Subsequently, the sprayed or coated core wasinserted into a die casting machine and the aluminum or other metal oflower melting point was cast under high pressure around the coated core.Upon removal of the completed casting from the machine the core wasseparated from the casting by-differential expansion, in the preferredembodiment, leaving a taperfree coated housing or casting of a highdegree of accuracy and smoothness which requires only a simple honingoperation on the interior of the casting to produce a smooth, accuratelydimensioned, wear resistant surface suitable for internal combustionengines either of the reciprocating or rotary type. I

While the iron alloycoating was sufficient for many applications, themore advanced engine cylinders and nomically, therefore, the compositeor dual layer coating of the present invention has a distinct advantage.

BRIEF STATEMENT OF THE INVENTION Briefly stated, the present inventionincludes a method for producing a coated die casting by depositing athin layer of very hard material such as tungsten or titanium carbide,on a die casting core, then applying a second, thicker backing layer ofmetal, such as steel on top of the hard metal layer and adhered theretoby the roughness of the exterior of the deposited hard metal layer. Thecoated core is then placed in a die casting machine and the cylinder orhousing is cast around it as in the previously explained transplant coatprocess. The coating adheres to the metal of the die casting byinterlocking of the casting metal with the rough exterior surface'of thebacking layer. The bond between the backing layer and the die casting isextremely good.

I BRIEF DESCRIPTION OF THE DRAWINGS I casting core suitable for use inthe manufacture of the housings have been'improved by the application ofa 1 hard metal layer, such as chromium, platedonto the V iron alloy. 7

It has also been known to apply a tungsten carbide layer directly ontothe interior wearing surface of an engine cylinder or housing after thecylinder or housing has been cast and machined. It is extremelydifficult to accomplish good adhesion between such a tungsten carbidelayer and the machined aluminum casting. In addition, this method is atime-consuming, wasteful process because the layer must be madeinitially quite thick and subsequently ground down to the finaldimensions, configuration and degree of smoothness. Grinding a tungstencarbide layer is difficult in itself. In addition, it has been foundthat a very thin tungsten carbide layer is undesirable because thecompressive force exerted thereon under service conditions at anoperating temperature of about 400F. will frequently cause a separationbetween the tungsten carbide layer and thealuminum of the casting.However, such separation can be prevented even when the tungsten carbidelayer is ve ry thin by employing a comparatively thick backing layer ofan iron alloy. The tungsten carbide material is quite expensive,currently selling for approximately $20.00 a pound while the iron layerpreviously used sells for only about $.40 a pound. E00- housing forrotary engine showing the spray application of an initial hard layersuch as tungsten or other metallic carbide on the core;

FIG. 2 is'a view similar to FIG. 1 showing the application of a layer ofa second metal backing the first hard layer;

. FIG. 3 is a diagrammatic view of the core in place in a die castingdie, the core bearing both of the previously applied coatings;

FIG. 4 is an elevational view of a completed casting with the coreremoved, leaving the coatings in place in the casting; and

FIG. 5 is an enlarged, fragmentary sectional view through the wall ofthe completed casting taken on the line 5-5 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT As indicated in the accompanyingdrawings, the present invention is most useful in the manufacture oftrochoid housings for rotary engines. As used herein the words cylinderand housing indicate cylinders for reciprocating engines and housingsfor rotary engines inter-changeably.

FIG. 1 of the drawings indicates diagrammatically at 10 a core for themanufacture of a housing for a rotary engine. The core 10 is preferablytaperfree so as to minimize the machining required to finish the finalcast housing. The core 10 is mounted for rotation on a lathe-likemachine and a metalizing gun 11 is mounted adjacent the machine to spraya layer 12 of a hard material, such as tungsten carbide, siliconcarbide, boron carbide, titanium carbide, chromium carbide or any ofseveral borides, usually associated with a binder such as cobalt,nickel, or the like-The layer 12 can also be of a ceramic material,e.g., zirconia or alumina, which can be applied without a binder, or canbe an alloy or a pure metal such as chromium, or molybdenum.Hereinafter, the various carbides, borides and the like are genericallyreferred to as hard metal; it is to be understood that this termencompasses the carbide and borides per se as well as in a particulateform dispersed in a binder such as nickel. The layer 12 can be formed byspraying, for example, a Metco 438 or 439 tungsten carbide nickelaluminide blend from the gun 11. In the preferred embodiment, thethickness of the hard metal layer 12 is from 0.003 to 0.020 inch, mostdesirably from 0.005 to 0.015 inch. Thicker coatings are equallyoperable but for economic reasons the thinnest possible coatingrequiring the minimum machining is desired. The hard metal layer adheressomewhat to the core but is not bonded thereto. If desired, the core 10may be coated with a solution or dispersion of a release agent such astalc or sodium bicarbonate prior to spraying on the initial hard metallayer. In an even more preferred embodiment an initial thin separationlayer of softer material, for example, of a ferrous or non-ferrous metalor alloy, is first deposited on the core. This separation layer can beiron, aluminum, zinc, tin or an alloy thereof and can be applied with agas or electric metalizing gun. Such a separation layer exhibits muchless tendency to adhere to the core than does the hard metal layer. Thisthin initial separation layer may be only from 0.001 inch to 0.003 inchthick, but it serves as an effective release layer with respect to thecore 10.

As shown in FIG. 2, the core having been coated with the hard metallayer 12 as in FIG. 1 is then subjected to spraying from a second gunindicated at 13 which deposits a thicker layer 14 of a material such assteel or a chromium-iron alloy such as is sold under the trade nameMetco No. 2. The layer 14 preferably has a thickness from 0.010 to 0.065inch, most desirably from 0.015 to 0.060 inch. The exterior of the hardmetal layer 12 sprayed on as in FIG. 1 is rough and pitted and the steellayer 14 applied as in FIG. 2 from the gun l3 adheres thereto byentering into the interstices of the rough and pitted surface of theexterior of the hard metal layer 12. This coating 14 itself exhibits onits exterior a rough and pitted surface as explained in my prior US.Pat. No. 3,083,424. The gun 13 can be a common gas or electricmetalizing gun.

The coated core 10, bearing both of the sprayed on layers 12 and 14deposited as in FIGS. 1 and 2, is then inserted into an appropriate diein a die casting machine diagrammatically indicated at 16 in FIG. 3. Thedie 16 has a cavity of an appropriate shape surrounding the core 10. Themetal of the housing is cast into the cavity of the die 16 under normaldie casting pressures which may run from 2000 psi. to 15,000 psi. Thehigh pressure exerted on the casting metal causes this metal to enterinto the interstices of the rough, pitted surface of the exterior of thelayer 14 of steel or other iron alloy. The completed casting is thenpermitted to solidify in the die 16 and is subsequently removed with thecore 10 still in place. The bond exhibited between the casting metal,which is preferably aluminum, and the coating 14 has a greater strengthof adherence than the bond between the initially deposited hard metallayer 12 and the core 10. Thus, when the core 10 is removed from thesolidified casting, as by a differential expansion, the coatings 12 and14 will adhere to the casting and not to the core. Attempts to removethe core from the casting by sheer physical force may result in adestruction of the coating 12 because of the adherence of some of thehard metal particles to the core which would then either score theremainder of the coating or cause its destruction for practical use.

Once the casting has been removed from the die casting machine and thecore 10 removed from the casting,

a section through the wall of the casting is essentially as shown inFIG. 5. FIG. 5 is, of course, an enlarged fragmentary view and shows therelatively thin internal hard metal layer 12 and the relatively thickerbacking layer 14 of the iron alloy interlocked to the hard metal layerat the interface 17. The aluminum or casting metal is interlocked as at18 to the backing layer 14 of the iron alloy. In each instance the bondbetween the layers, at 17 and 18, is only by reason of the interlockingof the material of each layer with its neighboring material in therough, pitted surface which forms the exterior of the layer after thespray coating application. There appears to be no chemical bonding.

If the technique has included the primary application of a release layerof iron alloy or the like the casting is then subjected to a machiningoperation, usually grinding, to expose the hard metal layer 12 which isdesired for the working surface of the housing.

The coated casting is then ground to its final finish and dimensions.Even though the hard metal layer may be ground down to a thickness ofonly a few microns in some areas it will not spall off or crack becausethe physical strength imparted by the backing layer of iron alloy issuch that even though the aluminum of the casting (having a highercoefficient of thermal expansion) tends to expand radially away from thecoatings upon heating during engine operation adequate strength of thecomposite layers is such that cracking does not occur.

I claim:

1. A method for producing a die casting which comprises applying to adie casting core a relatively thin hard metal coating having a rough andpitted exposed surface, applying to the hard metal coating, and intointerlocking engagement with the rough and pitted surface thereof, arelatively thicker backing coating of a metal which is strong, highlyadherent, and softer than said hard metal having a rough and pittedexposed surface, positioning the coated core in a die, introducing underdie casting pressure a metal having a lower melting temperature thanthat of either of the coatings into the die and into interlockingengagement with the rough and pitted surface of the metal coating on thecore, solidifying the introduced metal, and separating the core from thehard metal layer whereby the interior of the die casting exhibits thehard wear resistant character of the thin initial coating.

2. A method as claimed in claim 1 wherein the thickness of the hardmetal coating on the core is controlled to one sufficiently thin thatabsent the metal backing coating, a given compressive force appliedthereto when the finished casting is heated causes separation of thehard metal coating and the die cast metal, and the thickness of themetal backing coating is controlled to one at which the givencompressive force applied to the hard metal layer when the finishedcasting is heated does not cause separation.

3. A method as claimed in claim 1 wherein a dry particulate releaseagent is applied to the die casting core prior to application thereto ofthe layer of a hard metal.

4. A method as claimed in claim 1 wherein a metal coating is applied tothe die casting core as a separation layer prior to application theretoof the layer of a hard metal.

5. A method as claimed in claim 1 wherein a thin coating of a metal isfirst applied to the die casting core,

6. A method as claimed in claim wherein, after sep-- aration of the corefrom the casting, the metal coating that was first applied to the coreis removed by a machining operation to expose the hard metal coating.

7. A method as claimed in claim 4 wherein, after separation of the corefrom the casting, the metal coating that was first applied to the coreis removed by a machining operation to expose the hard metal coating.

1. A method for producing a die casting which comprises applying to adie casting core a relatively thin hard metal coating having a rough andpitted exposed surface, applying to the hard metal coating, and intointerlocking engagement with the rough and pitted surface thereof, arelatively thicker backing coating of a metal which is strong, highlyadherent, and softer than said hard metal having a rough and pittedexposed surface, positioning the coated core in a die, introducing underdie casting pressure a metal having a lower melting temperature thanthat of either of the coatings into the die and into interlockingengagement with the rough and pitted surface of the metal coating on thecore, solidifying the introduced metal, and separating the core from thehard metal layer whereby the interior of the die casting exhibits thehard wear resistant character of the thin initial coating.
 2. A methodas claimed in claim 1 wherein the thickness of the hard metal coating onthe core is controlled to one sufficiently thin that absent the metalbacking coating, a given compressive force applied thereto when thefinished casting is heated causes separation of the hard metal coatingand the die cast metal, and the thickness of the metal backing coatingis controlled to one at which the given compressive force applied to thehard metal layer when the finished casting is heated does not causeseparation.
 3. A method as claimed in claim 1 wherein a dry particulaterelease agent is applied to the die casting core prior to applicationthereto of the layer of a hard metal.
 4. A method as claimed in claim 1wherein a metal coating is applied to the die casting core as aseparation layer prior to application thereto of the layer of a hardmetal.
 5. A method as claimed in claim 1 wherein a thin coating of ametal is first applied to the die casting core, said metal having alower melting temperature than that of the hard metal, and the hardmetal coating is then applied to the metal coating.
 6. A method asclaimed in claim 5 wherein, after separation of the core from thecasting, the metal coating that was first applied to the core is removedby a machining operation to expose the hard metal coating.
 7. A methodas claimed in claim 4 wherein, after separation of the core from thecasting, the metal coating that was first applied to the core is removedby a machining operation to expose the hard metal coating.